Medical X-ray imaging devices usually include an X-ray source emitting X-ray radiation that passes through a patient's body part and a solid-state image sensor receiving the transmitted X-ray radiation to form an image of the body part. Many conventional X-ray imaging systems use a semiconductor charge-coupled device (CCD) as the image sensor. A layer of scintillator material is thus disposed in front of the CCD to convert the transmitted X-ray radiation into photoncarriers in a visible spectrum. Photogates in the pixel array of the CCD capture the visible photoncarriers and generate photoelectrons. A CCD signal representing a line-by-line scan of the image is produced by sequentially transferring the photoelectrons in the pixel array to a readout register.
X-ray imaging systems using CCD sensors are widely used in dental imaging applications, such as a compact device to photograph teeth. For example, U.S. Pat. Nos. 5,434,418 to Schick and 5,331,166 to Yamamoto et al. disclose this type of dental X-ray imaging devices. Both Schick and Yamamoto describe an automatic mechanism to trigger CCD readout at the end of exposure. Specifically, Schick uses a computer-aided electronic system to monitor photoinduced charges, i.e., a readout is triggered if the CCD or the photodiode array is no longer conducting. Yamamoto et al. devise an X-ray dose monitoring system to control the exposure of the X-ray. Auxiliary X-ray detectors located on the outer casing of the image sensor are used to generate a control signal to shut off the X-ray source and trigger a CCD readout if the X-ray dose reaches a predetermined dose value.
However, the triggering of the X-ray source is asynchronous with respect to the operation of the image sensor in many prior-art systems. One conventional method in operating the CCD sensor is to continuously read out the sensor. The X-ray source is triggered manually by the operator. The X-rays go through the patient's teeth and strike the sensor. When "bright" pixels are detected, a full frame of data is read out and captured. A dark reference frame, captured either during a calibration step or close in time (e.g., less than 1 second) to the illuminated frame, is subtracted from the image to produce the digital X-ray image with minimized background noise.